The Devonian Mahantango Formation is a mapped bedrock unit in Pennsylvania, West Virginia, and Maryland.

[6] The Mahantango Formation is a gray, brown, and olive siltstone and shale,[7][8] characterized by coarsening-upward cycles.

[11] In south-central Pennsylvania, the Mahantango includes the Clearville, Frame, Chaneysville, and Gander Run Members.

There are numerous marine fossils found in the Mahantango including brachiopods, crinoids, trilobites, bivalves, and bryozoans.

Atactotoechus furcatus Relative age dating of the Mahantango places it in the Middle Devonian period, being deposited between 392 and 385 (±3) million years ago.